Composite articles may be produced by mechanical forming or method hot drape forming (HDF). When producing composite articles by hot drape forming, it requires the use of pressure onto stacks of prepregs or plies against a tool. A vacuum bag is used to achieve a pressure such that wrinkles and trapped air in the stack can be forced out under vacuum pressure. The vacuum bag can be used for curing the resin of the plies in an autoclave or in an oven at elevated temperature and pressure. After curing the bag is removed from the forming tool and the article is removed from the tool. By mechanical forming a roll could be used to conform a stack of prepregs or plies against a tool.
When articles are hot drape formed or mechanical formed on a tool, which has one substantially longitudinal direction and one substantially transversal direction, wrinkles are easily formed if the tool has some small curvature, bevels, any protrusion or any depression formed in the tool. It is thus desirable that formation of wrinkles at the location of bevels or other projections or depressions of the article is eliminated in the stack during the forming process.
The tool may have a pad-up area, or a depressed area in the tool. The purpose of the depressed area or the pad-up area is to make a thickening of the article. The thickening is a reinforcement in the article. It is difficult to form articles without wrinkles on a tool which has pad-up areas or depressed areas.
U.S. 2004/0115299 discloses a method where the vacuum bag is made thinner within the area of a forming surface, which more readily stretches and conforms the stack to the forming surface.
U.S Pat. No. 5,129,813 describes an evacuation process which reduces the tendency of the vacuum bag to trap air. The method provides a vacuum bag having a plurality of interconnected channels.
U.S. 2005/0163975 A1 and U.S. Pat. No. 5,984,511 relates to stacking sequences when forming articles having an uneven surface, but does not relate to the problem of wrinkles in the article.
EP 1775106 describes forming articles on tools, which may have a curvature, which relates to the orienting of fibre directions of the prepregs in the fibres stack so that none of the fibre directions of the prepregs occur parallel with a curve of the moulding tool. The downforming may then be carried out in a simpler manner.
The applicant has found a method in which it is possible to hot drape form an article on a tool without creating wrinkles in the article. This is disclosed in PCT/SE2008/051304 (not yet published) which relates to a method in which a certain stacking sequence is used for avoiding wrinkling when forming an article by hot drape forming. Plies having fibres in the longitudinal direction of the tool, plies having fibres in the orthogonal direction of the tool and fibres having a diagonal direction of the tool are arranged in a specific order in the stack in order to avoid wrinkles. The inventors of the present invention have now evaluated the stacking method disclosed in PCT/SE2008/051304. In some cases, there might be some problems during the forming in which the article becomes thinner in certain parts. The article can for example be thinner in curved parts. The present invention is thus an evaluation of the stacking method above and the present invention leads for example to fewer mistakes during production of composite articles.